Heterogeneous Degradation and Recyclability of PP Studied by Imaging Chemiluminescence
Abstract: Oxidative degradation is a serious threat to long-term use and recyclability of polymers. Successful recycling of polymeric quality products requires characterization of the polymer material after reprocessing and use. Lately, a growing awareness of the heterogeneous nature of degradation has developed in terms of physical spreading, non-uniform distribution of stabilizer, physical loss of stabilizer etc. In this thesis the issues of heterogeneous degradation and recyclability of PP are addressed mainly using imaging chemiluminescence (ICL) technique. An extensive test program was carried out on PP subjected to simulated recycling in terms of combined (re)processing and oven ageing or alternatively to repeated reprocessing steps or continuous oven ageing. For this study an ICL instrument with improved performance was developed including a multi-cell specimen chamber with up to 23 cells with excellent control of temperature and gas flow in the cells and a high sensitivity for low-mass specimens. Comparative studies with microcalorimetry (MC) showed good agreement between CL intensity-time and heat flow-time curves during oxidation of PP. From ICL images, oxidation induction time (OIT) and total luminous intensity (TLI) data as well as from data of other techniques like elongation at break, yellowness index and melt viscosity a number of conclusions were made: The total oven lifetime was reduced if oven ageing included an intermediate reprocessing step. A commercial stabilization system based on a low-molar mass phenolic antioxidant was inadequate for recycling due to loss by evaporation, while an antioxidant with twice the molar mass showed satisfactory performance. The effect of upgrading PP during reprocessing with virgin PP was small, while addition of extra stabilizer prolonged the oven lifetime. In mapping the heterogeneity of the degradation of PP plaques pronounced edge oxidation was discovered and explained by antioxidant loss by evaporation. Simultaneous ICL and oxygen uptake experiments showed matching OITs and supported the opinion that luminescence and oxygen uptake reflect the oxidizing area rather than the oxidation kinetics.
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